In 2000 the United States Congress enacted a bill under the title Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act. One aspect of this law called for the Department of Transportation to initiate rulemaking related to tire inflation. Proposed rulemaking requires implementation of a warning system in new motor vehicles that indicates to the operator when a tire is significantly under-inflated.
Two types of Tire Pressure Monitoring (TPM) systems have been developed in response to this legislation. These are direct and indirect systems. Direct systems generally operate with a tire pressure sensor in each tire cavity. Indirect TPM systems generally monitor under-inflation by comparing characteristics of tires such as wheel speed. Each such system has limitations. It has now been discovered that a hybrid TPM system offers improvements over these known methods of tire pressure measurement.
One shortcoming of the presently known systems relates to starting tire pressure. An indirect system requires that all of the tires be filled to their nominal pressures. Vehicle manufacturers recommend that tires be filled to placard (for example, 30 PSI) pressure when cold. Typically this requires that a vehicle remain stationary for at least 1 hour. After a vehicle has been driven for more than 20 minutes at speeds between 50 and 100 KPH, manufacturers recommend adjusting all tire pressures to placard plus an additional 4 PSI. This additional pressure accommodates for the friction related heat and pressure increase that occurs in all tires while driving.
Both prior art systems allow tire characterization anytime the reset function or switch is properly deployed even if the tires are not at nominal pressures. The shortcoming lies in the fact that those systems have no direct way to ensure proper inflation levels before starting the characterization procedure. For example, in an indirect system actual pressures that are significantly off of recommended inflation levels can yield the same ratios as do tires with proper inflation. In that case underinflated tires are indistinguishable from each other. The following cases illustrate how differing inflations can nevertheless have similar ratios. (Here, and elsewhere in the application, the convention is used where LF, RF, LR, and RR mean, respectively left front, right front, left rear, and right rear in a conventional, four tire automobile arrangement.)
CASE 1CASE 2LF = 18RF = 17.25LF = 30.6RF = 29.3LR = 18.5RR = 20LR = 31.4RR = 34
In the above example, the system will compare pressures in each tire. The pressure ratios of both Case 1 and Case 2 are similar. Thus the driver will not be warned of a low pressure situation. This represents a worst-case scenario for indirect systems. It relies on the customer to ensure that the tire pressures are optimal before executing a tire characterization procedure.
Hence there is a need for an improved tire pressure measurement system. There is a need for a pressure measurement system that can determine tire pressure at any time, regardless whether the car has been running or not, and regardless whether the actual tire pressure is greater than or less than the placard pressure. Additionally there is a need for a pressure measurement system that individually reports pressure for each of four tires. Further there is a need for a pressure measurement system that visually reports pressure readings to a driver/user. Finally, there is also a need for an improved TPM system that integrates into present operator equipment such as key fobs and remote keyless entry (RKE) systems. The present invention addresses one or more of these needs.